The present invention relates to improved electrodes particularly adapted for use as anodes in electrochemical process involving the electrolysis of brines.
A variety of materials have been tested and used as chlorine anodes in electrolytic cells. In the past, the material most commonly used for this purpose has been graphite. However, the problems associated with the use of graphite anodes are several. The chlorine overvoltage of graphite is relatively high, in comparision for example with the noble metals. Furthermore, in the corrosive media of an electrochemical cell graphite wears readily, resulting in substantial loss of graphite and the ultimate expense of replacement as well as continued maintenance problems resulting from the need for frequent adjustment of spacing between the anode and cathode as the graphite wears away. The use of noble metals and noble metal oxides as anode materials provides substantial advantages over the use of graphite. The electrical conductivity of the noble metals and oxides is substantially higher and the chlorine overvoltage substantially lower than that of graphite. In addition, the dimensional stability of the noble metals and noble metal oxides represents a substantial improvement over graphite. However, the use of noble metals or oxides thereof as a major material of construction in anodes results in an economic disadvantage due to the excessively high cost of such materials.
In attempts to avoid the use of the expensive noble metals various other anode materials have been proposed for use as coatings over valve metal substrates. In German Offenlegungsschrift No. 2,210,065, for example, it is disclosed that perovskite compositions, including oxide bronzes, can be formed as coatings on a valve metal substrate to form an anode useful in electrochemical processes.
The oxide bronzes are non-stoichiometric compounds which may be characterized by the formula A.sub.x BO.sub.3 where A may be an alkali metal, an alkali-earth metal, a lanthanide, or other, generally having an ionic radius of between about 0.60 and about 1.40A; B is typically tungsten, niobium, molybdenum, tantalum, titanium or vanadium; and x may range from greater than 0 to less than 1.0. The oxide bronzes were first reported (as tungsten bronzes) by Wohler in 1824. They are neither alloys nor intermetallic compounds. The term "bronze" was originally applied to describe the yellow, metallic bronze-like luster of Na.sub.x WO.sub.3 where x is about 0.8 or 0.9. However, the term bronze or oxide bronze is now generally applied to this type of composition regardless of color or luster and, depending on the identity of the B metal ion in the formula A.sub.x BO.sub.3 may be variously referred to as tungsten bronze, vanadium bronze, or the like. The oxide bronzes are reported as possessing excellent chemical stability and unusually high electrical conductivity ranging from that of a semiconductor to that of a metal.
In the electrolytic production of chlorine, anodes of oxide bronze provide the advantage of economy in the elimination of the use of expensive noble metals. However, the oxide bronze compositions although useful as an anode material exhibit a chlorine overvoltage that is substantially higher than that of the noble metals or noble metal oxides. Thus, despite the elimination of expensive noble metals, the cost of chlorine production, in processes using such anodes, is relatively high. It has further been proposed, according to British Pat. No. 1,164,477, to prepare electrodes of tungsten bronze by heating and vaporizing metallic tungsten, sodium tungstate and tungsten trioxide then allowing the vapors to deposit on a cooled metallic plate. The sodium tungsten bronze may then be used as formed or may be pulverized and sintered in any shape or a metal such as platinum, palladium, iridium, rhodium or silver may be electrodeposited on it. The electrodes are especially useful in processes such as the electrolytic oxidation of methanol in an acidic solution. However, the use of such an electrode as an anode in chlor-alkali cells presents certain problems. Firstly, it has been found that coatings of noble metals such as platinum, ruthenium, or palladium deteriorate in the anodic environment of a chlor-alkali cell at an undesirable rate. For example, the deterioration of such noble metal coatings is substantially faster than that of coatings of the noble metal oxides. Secondly, despite statements in the literature indicating a high degree of chemical stability of alkali metal tungsten bronzes, especially in acid environments, it has been found that, in the anodic environment of a chlor-alkali cell, these compositions deteriorate at a rate which, although much slower than many materials, e.g., steel, copper, and the like, is nevertheless uneconomical and precludes their commercial use as a primary operative anode material for such cells. If such compositions are employed as coatings over a metal substrate, the substrate is ultimately exposed to the anode environment.
Considerable effort has been expended in recent years in attempts to develop improved anode materials and structures utilizing the advantages of noble metals or noble metal oxides. A great amount of effort has been directed to the development of anodes having a high operative surface area of a noble metal or noble metal oxide in comparison with the total quantity of the material employed. This may be done, for example, by employing the noble metal as a thin film or coating over an electrically conductive substrate. However, when it is attempted to minimize the aforementioned economic disadvantage of the noble metals by applying them in the form of very thin films over a metal substrate, it has been found that such very thin films are often porous. The result is an exposure of the substrate to the anode environment, through the pores in the outer layer. In addition, in normal use in a chlor-alkali cell, a small amount of chemical attack, wear, spalling or flaking off of portions of the noble metal or noble metal oxide is likely to occur, resulting in further exposure of the substrate. In general such problems are somewhat more severe with respect to coatings of noble metals than with noble metal oxides. Many materials, otherwise suitable for use as a substrate are susceptible to chemical attack and rapid deterioration upon exposure to the anode environment. In an attempt to assure minimum deterioration of the substrate under such circumstances, anode manufacturers commonly utilize a valve metal such as titanium as the substrate material. Upon exposure to the anodic environment, titanium, as well as other valve metals, will passivate, that is, form a surface layer of oxide which serves to protect the substrate from further chemical attack. The oxide thus formed, however, is non-conductive and is not catalytically active. While this passive oxide layer serves to protect the exposed areas of metal, some chemical attack can take place during long term use and cause dissolution of substrate metal and undercutting of the coating. As a result the operative surface area of the anode is decreased, and the efficiency of the cell is lowered. The more rapidly such passivation occurs, the more frequently the anode must be replaced.
Accordingly, it is an object of the present invention to provide improved electrodes for use as anodes in electrolytic processes. It is a further object to provide such anodes having an operative surface of noble metal oxide and having improved maintenance characteristics.